Accumulation and vertical distribution of glomalin-related soil protein in French temperate forest soils as a function of tree type, climate and soil properties

Soil organic matter (SOM) is of critical importance to the functioning of ecosystems because of its effects on soil nutritional quality, microbial activity and soil structure. SOM is made up of a variety of compounds ranging from chemically reactive and mobile forms to stable, recalcitrant forms. An operationally defined fraction of soil organic matter known as glomalin-related soil protein (GRSP) is thought to be of fungal origin, very stable and responsible for increased physical stability of soil. To date, the origin of this fraction of SOM and the factors controlling its accumulation in soils are not well understood. The aim of this study was to contribute to the understanding of the origin of this fraction and the factors which contribute to its accumulation in the soil. This study was carried out on archived soils from 102 sites of the long-term monitoring network for forest ecosystems in France (RENECOFOR), from depths up to 1 m. The effects of tree type, climate and soil properties on the accumulation and vertical distribution of GRSP were assessed. GRSP was greater under conifer forests than broadleaf forests. GRSP in the top soil layer was positively correlated with both SOC and total nitrogen, and inversely related with pH, with the strongest effect observed at very acid (pH <= 4.0). Both GRSP and SOC were found to follow exponential declines with depth. The contrast in vertical distribution of GRSP and SOC supports the hypothesis that GRSP is a distinct fraction of SOM. Climate and soil properties had significant influence on the vertical distribution of GRSP. The accumulation of GRSP, and the enrichment of SOC in GRSP, at acid pH could indicate a fungal origin of GRSP.